Neurostimulation is used effectively today to treat several diseases by placing electrodes in contact with neural tissue. Medical devices used in the course of neurostimulation generally transfer one or more of electric charge and electric fields to tissue, resulting in physiological change, which benefits the patient, or performs a physiological measurement. For example, electrical neurostimulation is used in the cochlea to produce responses similar to those produced from audible sounds. As another example, electrodes are placed near an animal's spine and configured to generate electrical pulses to treat pain. As another example, electrodes are placed in the deep brain for stimulation neurological targets including the subthalamic nucleus, the globus pallidus, configured to generate electrical pulses to treat the symptoms of movement disorders, such as Parkinson's disease, Essential Tremor or Dystonia. Such therapies may also treat the symptoms of Epilepsy and other neurological disorders. Neurostimulation is also used in other parts of the body, such as the retina, and the peripheral nervous system.
The localization of such electrical stimulation is important, and leads to higher efficiency in the therapy. Higher localization of the electrical stimulation generally requires smaller electrodes. The smaller electrodes exhibit particular electrical characteristics once placed into contact with an electrolyte such as the physiological fluid in the body.
The stimulation signals used in electrical stimulation can be fully described by their amplitude, pulse shape, and pulse frequency. Signal amplitudes are generally measured in units of voltage or current. Pulse shapes are generally described by their geometric shape and pulse width. For example, a commonly used pulse shape is a rectangular pulse with a pulse width, measured in units of time, such as micro-seconds. Finally, pulse repetition frequency generally describes the number of pulses per second applied to the electrodes. For example, a rectangular pulse of width 50 micro-seconds can be applied to an electrode at a frequency of 130 Hz. A suitable combination of amplitude, pulse shape, and pulse repetition frequency providing effective treatment is generally difficult to determine.
Several attempts to increase stimulation efficiency have been made. The methods used, however, have a direct effect on power consumption, tissue narcosis, and would potentially degrade the electrode materials due to corrosion. Empirical and simulation methods have been used to find a stimulation amplitude “threshold” at a particular frequency, such as 1 kHz or 10 kHz. Threshold determination techniques are explained by Palanker et al. and Jensen et al. empirically in the case of retinal stimulation.
The electrical stimulation of tissue with micro-scale electrodes presents several problems that have been previously identified, but have not been properly addressed. First, the interface impedance between a microelectrode and the surrounding tissue is extremely high, usually on the order of 1 MO for a 50 diameter electrode at biologically significant frequencies of 1 kHz. Such a high impedance leads to a high current requirement in order to achieve a sufficient voltage across the neural tissue for activation. Such high current can destroy the electrode material because it is susceptible to corrosion in the generally electrolytic environment of physiological fluid. Such corrosion would be undesirable as dangerous toxins can be released into the tissue. Furthermore, high currents will quickly decrease battery life for implantable devices.